1. Field of the Invention
The invention relates generally to light transmissive optical films, and more particularly, the invention relates to an optical film with an array of variable angle prisms.
2. Description of the Related Technology
Backlit display devices, such as liquid crystal display (LCD) devices, commonly use a wedge-shaped lightguide. The wedge-shaped lightguide couples light from a substantially linear source, such as a cold cathode fluorescent lamp (CCFL), to a substantially planar output. The planar output is then coupled to the LCD.
The performance of a display device is often judged by its brightness. From a subjective standpoint relatively small increases in overall brightness are not easily perceived by the end user of the display device, but it is possible to measure relatively small increases in brightness objectively. While not directly appreciated by the end user, a display with an objectively measured increase in overall brightness of only a small percentage, for example, perhaps as little as 1 percent, is perceived as being significantly better by the designer of the product using the display. This is because the designer can allocate less power to the display device, yet still achieve an acceptable level of brightness. For battery powered, portable devices, this translates to longer running times.
The alternatives for increasing display brightness include using more or brighter light sources. Counter to the ability to decrease the power allocation to the display device, additional light sources and/or brighter light sources consume more energy, which for portable devices this correlates to decreased battery life. Also, adding light sources to the device may increase the product cost and can lead to reduced reliability of the device.
Brightness is also enhanced by more efficiently using the light that is available within the display device, i.e., to direct more of the available light within the display along a preferred viewing axis. A number of mechanisms have been employed within display devices to improve display device efficiency. For example, brightness enhancing films having prismatic structures are frequently used to direct light that would otherwise not be viewed along the viewing axis. A typical flat panel display device may use several different films to provide an overall bright, high contrast display with substantially uniform output along the preferred viewing directions. Surface diffusers or bulk diffusers are sometimes used to mask defects in the output of the lightguide, but most diffusers scatter light from the viewing axis and therefore reduce on-axis brightness.
Lightguide improvements have also contributed to improved brightness in display devices. Typical lightguides extract light by diffusion and may be enhanced by geometric recycling. Light rays entering the lightguide encounter diffusing elements, typically a pattern of white dots applied to a surface of the lightguide, and are diffusively extracted by scattering from the lightguide. Other light rays are totally internally reflected within the lightguide until encountering a diffusing element. Losses are encountered in these processes, and because the light is diffusely extracted, without any collimation, on-axis brightness is lower. With enhancement, the diffuse light rays may be directed more on axis, in a quasi-collimation process, which results in enhanced on-axis brightness.
Another method of extracting light from a lightguide is by use of frustrated total internal reflection (TIR). In one type of frustrated TIR the lightguide has a wedge shape, and light rays incident on a thick edge of the lightguide are totally internally reflected until achieving critical angle relative to the top and bottom surfaces of the lightguide. These sub-critical angle light rays are then extracted, or more succinctly refract from the lightguide, at a glancing angle to the output surface. To be useful for illuminating a display device, these light rays must then be turned substantially parallel to a viewing, or output, axis of the display device. This turning is usually accomplished using a turning lens or turning film.
A turning lens or turning film typically includes prism structures formed on an input surface, and the input surface is disposed adjacent the lightguide. The light rays exiting the lightguide at the glancing angle, usually less than 30xc2x0 to the output surface, encounter the prism structures. The light rays are refracted by a first surface of the prism structures and are reflected by a second surface of the prism structures such that they are directed by the turning lens or film in the desired direction, e.g., substantially parallel to a viewing axis of the display.
An optical effect resulting from frustrated TIR lightguides not adequately compensated for in presently known display devices is referred to as ripple. Ripple is a periodic fluctuation in the luminance of the wedge light output. The fluctuation amplitude, spatial frequency and phase are largely determined by the wedge angle, input aperture and lamp coupling to the input aperture. The emergence angle and the location of emergence from the wedge maps directly onto a position on the input aperture and an emission angle from the input aperture. Thus, nonuniformities in the spatial and angular components of emission from the input aperture map into corresponding changes in the wedge output brightness. The result is an effect wherein the display has a bright and dark banded appearance parallel to the light source. The effect is most perceptible nearest to the lightguide entrance, but may be observed over the entire output surface.
Attempts to correct ripple, or essentially mask the appearance of ripple, include adding some form of optical power or scattering to the wedge structure to try and provide more uniform light extraction. However, these attempts have not proven entirely satisfactory.
In accordance with the invention, an optical film useful for reducing or eliminating ripple (luminance non-uniformity) is formed with an array of turning prisms formed on an input surface. The prisms are contained within the array in clusters of prisms that include prisms of two or more prism configurations.
In a first aspect of the invention, an optical turning film has a first surface including an array of prisms. The array has a plurality of first prisms, with each of the first prisms having a first prism configuration, and a plurality of second prisms, with each of the second prisms having a second prism configuration different from the first prism configuration. The optical film also has a second surface opposing the first surface, and light rays directed at a glancing angle to the first surface are directed by the plurality of first prisms and the plurality of second prisms through the optical turning film and are emitted from the second surface at an angle substantially parallel to a viewing direction of a display.
In another aspect of the invention, an optical turning film is a sheet of light transmissive optical film having a first surface and a second surface. A plurality of first light redirecting prisms and a plurality of second light redirecting prisms are formed in the first surface. The plurality of first light redirecting prisms and the plurality of second light redirecting prisms are arranged in an array of prisms on the first surface. The first light redirecting prisms have a first prism configuration, and the second light redirecting prisms have a second prism configuration, different from the first prism configuration. The plurality of first light redirecting prisms and the second plurality of light redirecting prisms are organized into a plurality of first prism groups and a plurality of second prism groups. The first prism groups and the second prism groups are arranged in the array in a pattern of first prisms and second prisms.
In still another aspect of the invention, an illumination device has a lightguide with an input surface and an output surface. Light rays incident on the input surface are refracted into the lightguide and propagate within the lightguide by TIR. These light rays then exit the output surface via frustrated TIR at a glancing angle to the output surface. A light source is coupled to the input surface to project the light rays on the input surface, and a turning film is disposed adjacent the output surface. The turning film has a first surface and a second surface opposing the first surface. An array of prisms is formed in the first surface of the turning film, and the array of prisms has a plurality of first prisms having a first prism configuration and a plurality of second prisms having a second prism configuration different from the first prism configuration. The array of prisms are arranged to redirect the light rays from the glancing angle to an angle substantially aligned with a viewing axis of the illumination device.